Conventionally, boilers provided in thermal power stations and a variety of large-scale boilers employing a fuel such as petroleum, coal, or fuel gas, waste incinerators, and similar apparatuses have been equipped with a flue gas NOx removal apparatus which contains a plurality of NOx removal catalyst layers.
The NOx removal catalyst is generally composed of a carrier (e.g., TiO2), an active component (e.g., V2O5), and a co-catalyst component (e.g., tungsten oxide or molybdenum oxide), and multi-component oxide NOx removal catalysts such as VOx—WOy—TiO2 and VOx—MoOy—TiO2 are employed.
The NOx removal catalysts typically assume the form of honeycomb, plate, etc. Honeycomb-form catalysts include a coated catalyst, which is fabricated by producing a honeycomb substrate and coating the substrate with a catalyst component; a kneaded catalyst, which is fabricated by kneading a substrate material with a catalyst component and molding into a honeycomb catalyst; and an impregnated catalyst, which is fabricated by impregnating a honeycomb substrate with a catalyst component. Plate-form catalyst are fabricated by coating a metallic substrate or a ceramic substrate with a catalyst component.
In any case, during use, the catalytic performance of the above catalysts is problematically deteriorated with elapse of time as a result of deposition, on the surface of the catalysts, of a substance which deteriorates the catalytic performance (hereinafter referred to as deteriorating substance) or through migration of the dissolved deteriorating substance into the catalysts.
In this connection, a variety of methods for regenerating an NOx removal catalyst have conventionally been studied.
For example, there have been studied some methods including physically removing a deteriorated portion and foreign matter so as to expose a catalytically active surface; e.g., a method including abrasion an inner surface of a discharge gas conduit by use of an abrasive (Japanese Patent Application Laid-Open (kokai) No. 1-119343); a method including scraping a deteriorated surface portion of an NOx removal catalyst to thereby expose a catalytically active new surface (Japanese Patent Application Laid-Open (kokai) No. 4-197451); and a method including causing a gas accompanying microparticles to flow through a through-hole to thereby remove foreign matter (Japanese Patent Application Laid-Open (kokai) No. 7-116523).
In addition, there have been studied catalytic performance regeneration methods through washing; e.g., a method including washing a deteriorated catalyst with an acid (pH≦55) or an alkali (pH≧8) (Japanese Patent Application Laid-open (kokai) No, 64-80444); a method including washing a deteriorated catalyst sequentially with water or a dilute aqueous inorganic acid solution, with a 0.1 to 5 wt. % aqueous oxalic acid solution, and with water to remove oxalic acid residing on the catalyst (Japanese Patent Application Laid-Open (kokai) No. 7-222942); and a method including washing a deteriorated catalyst with water (50° C. to 80° C.), followed by drying (Japanese Patent Application Laid-Open (kokai) No. 8-196920).
However, methods based on physical abrasion or a similar technique have drawbacks in that operation is cumbersome and that an NOx removal catalyst itself is cracked or broken during a regeneration process.
In the case of washing an NOx removal catalyst, an alkaline component is removed through washing with an aqueous alkaline solution, hot water, etc., and heavy metal components predominantly containing vanadium are effectively removed through washing with an aqueous oxalic acid solution. However, even though these washing-based approaches are employed, washing out the deteriorating substances would be insufficient. Thus, washing-based regeneration methods employing a variety of cleaning components have been studied.
For example, the following regeneration methods have been proposed:                a method for regenerating activity of an NOx removal catalyst exhibiting a percent conversion to SO3 that is elevated through deposition of vanadium, wherein the catalyst is washed with a 0.1 to 30% aqueous citric acid solution and a 0.1 to 20% aqueous sulfuric acid solution (Japanese Patent Application Laid-Open (kokai) No. 10-156192);        a method for regenerating an NOx removal catalyst having lowered NOx removal performance, wherein the catalyst is washed with a cleaning liquid having a hydrofluoric acid concentration of 0.3 to 3 wt. % at a constant temperature of 20 to 80° C. (Japanese Patent Application Laid-Open (kokai) No. 10-235209);        a method for regenerating activity of an NOx removal catalyst exhibiting percent NOx removal lowered by deposition of Na and K originating from a boiler employing heavy oil as a fuel and exhibiting a percent conversion to SO3 that is elevated through adhesion of vanadium contained in the fuel, wherein the catalyst is washed with an aqueous inorganic alkaline solution and an oxidizing agent solution (Japanese Patent Application Laid-Open (kokai) No. 10-156193);        a method for regenerating an NOx removal catalyst containing a vanadium compound and titanium oxide, the catalyst having been deteriorated by sulfur oxide, wherein the activity-deteriorated catalyst is brought into contact with an aqueous solution containing a substance which generates ammonia in the presence of acid or by heat, and the mixture is heated or treated with acid, followed by drying and firing in air (Japanese Patent Application Laid-Open (kokai) No. 2000-107612); and        a method for regenerating an NOx removal catalyst exhibiting NOx removal performance lowered through poisoning by an arsenic compound contained in a discharge gas from a boiler employing coal as a fuel, wherein the catalyst is immersed in a cleaning liquid having a sulfuric acid concentration or an ammonia concentration of 0.05 to 20 wt. % at a constant temperature of 10 to 90° C., followed by washing the catalyst with water (Japanese Patent No. 3059136).        
Treatment of the wastewater after washing the catalyst has also been studied, and some wastewater treatment methods have been proposed. For example, in a method for regenerating activity of an NOx removal catalyst on which vanadium has been deposited, which method includes washing the catalyst with a cleaning liquid including an aqueous organic acid solution, an aqueous inorganic acid solution, or an aqueous alkaline solution, there has been proposed a method for regenerating wastewater including removing eluted vanadium ions from a wash wastewater through ion exchange (Japanese Patent Application Laid-Open (kokai) No. 10-33482). In regeneration of performance of an NOx removal catalyst exhibiting NOx removal performance lowered through employment in NOx removal from a combustion discharge gas from a boiler and SO2 oxidation performance elevated through the same, by regeneration treatment of the catalyst in water or an aqueous oxalic acid solution, there has been proposed a method of reusing the regeneration water or regeneration aqueous oxalic acid solution, wherein the water or aqueous oxalic acid solution is fed into a furnace of the above boiler (Japanese Patent No. 2994769).
As described above, there have still been studied a variety of methods for regenerating an NOx removal catalyst including washing the NOx removal catalyst and methods for treating wastewater produced by washing of the catalyst. However, all these methods are still unsatisfactory.
In view of the foregoing, an object of the present invention is to provide a method for regenerating an NOx removal catalyst, the method being capable of readily restoring the catalytic activity of a deteriorated NOx removal catalyst; being performed by simple operations; and attaining high operational efficiency.